[Test] Reset ApplicationManager.application when shared application environments are disposed

- Shared application environments can be disposed in the middle of test
  plan execution when the project count reaches zero. So resetting
  `ApplicationManager` only on "test plan execution finished" is
  incorrect, because it does not account for that scenario. Instead, we
  should reset `ApplicationManager` every time the application
  environment is disposed.
- We should also keep the manual reset in `testPlanExecutionFinished`
  because `disposeApplicationEnvironment` may not need to dispose
  anything if there isn't a shared application environment, but we
  should still reset `ApplicationManager` if an unshared application
  environment was created.
- This fixes "Some test disposed, but forgot to clear MockApplication"
  exceptions which appeared after KT-64099 because it opened up proper
  disposal of the shared application environment (instead of it being in
  a state of leakage because not all project disposables were properly
  disposed).

^KT-63650
^KT-64099

compiler/tests-common/tests/org/jetbrains/kotlin/test/testFramework/TestApplicationUtil.kt

@@ -6,9 +6,8 @@
 package org.jetbrains.kotlin.test.testFramework
 
 import com.intellij.openapi.application.Application
-import com.intellij.openapi.application.ApplicationManager
+import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
 import org.jetbrains.kotlin.utils.rethrow
-import java.lang.reflect.Field
 
 fun resetApplicationToNull(old: Application?) {
     if (old != null) return
@@ -17,9 +16,7 @@ fun resetApplicationToNull(old: Application?) {
 
 fun resetApplicationToNull() {
     try {
-        val ourApplicationField: Field = ApplicationManager::class.java.getDeclaredField("ourApplication")
-        ourApplicationField.setAccessible(true)
-        ourApplicationField.set(null, null)
+        KotlinCoreEnvironment.resetApplicationManager()
     } catch (e: Exception) {
         throw rethrow(e)
     }